Almost 10 percent of the adult US population have been suggested to suffer from a depressive illness with large economic consequences in addition to human suffering. A growing number of studies have pointed to the involvement of disorders of the hypothalamic-pituitary-adrenal (HPA) axis in the etiology and symptomatology of major depressive disorders. This proposal focuses on the paraventricular nucleus of the hypothalamus (PVN) as the key final common integration site and output pathway of the HPA axis. The PVN is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. Nuclear development, similar to other brain regions, proceeds through several key developmental phases that can be characterized simply by the generation of neurons, the migration of neurons to proper positions, the choice of life or death for new neurons, the establishment of cell phenotypes, and finally the establishment of functional connections. Our current and proposed research plan tests hypotheses for each of these facets of PVN development as they may contribute as fetal antecedents to adult dysfunction that may include susceptibility to major depressive disorder. PVN formation and function depends upon the expression of a number of transcription factors on one hand and selected effector molecules on the other. There are two basic hypotheses for mechanisms by which this influence is mediated. One suggests that these transcription factors are critical for the terminal differentiation of PVN neurons that remain in their normal locations. By contrast, we hypothesize that there are alterations in PVN differentiation that are secondary to alterations in the positions of PVN neurons and that a number of factors are critical for determining normal neuronal migration in the region of the PVN. Our specific aims will test several hypotheses concerning the role of several effector molecules in different stages of PVN development. We will ask what is the pattern of cell migration to the PVN and then how nitric oxide, gamma-aminobutyric acid (GABA), and brain-derived neurotrophic factor (BDNF) may play specific roles in nuclear organization. We will ask these questions in the context of potential differences between males and females to determine how sex differences in the developing HPA axis might contribute to the greater risk of depression in females.